eldest son; first-born son
Refers to the oldest son in a family. May be used even when he is the only son.
彼は長男として家を継ぐことになった。
As the eldest son, he was to succeed the family.
長男は来年大学を卒業します。
My eldest son will graduate from university next year.
His eldest son was born last year.
He was raised strictly as the eldest son.
In this family, it's expected that the eldest son will take over the family business.